This invention relates to the field of separation in which different fluids are separated by a permeable membrane so that one or more components of one fluid will pass through the membrane to the other fluid, such as in dialysis and desalination. For purposes of illustration the disclosed and claimed invention related particularly to an artificial kidney system for treating human blood and more specifically to improvements in coil type dialyzers.
There are several kinds of commercial coil type dialyzers. However, their basic principle of operation and structure are similar. Coil type dialyzers are made of semi-permeable membrane tubing, such as Cuprophan, (a trademark of Enka Glenstaff, West Wuppertal, Germany, for regenerated cullulose dialysis tubing), wound around a hollow core along with a spacing or support material. This support material is usually made of a plastic mesh, netting or thin sheet with numerous surface irregularities on one side or both. This support material maintains a relatively uniform clearance between layers of membrane tubing wound around the core, and prevents the membrane tubing layers from contacting one another.
The core with the membrane tubing and support material spirally wound thereon is placed in a casing which has openings at the bottom for dialysate fluid inlet and openings at the top for dialysate fluid outlet. Generally, the fluid to be dialyzed (perfusate) enters through one end of the semi-permeable tubing flows through the inside of the tubing and exits the other end of the membrane tubing. On the other hand, dialysate fluid flows between and around the outside of the membrane tubing and continuously removes permeating solutes. These solutes come from the perfusate by dialysis across the semi-permeable membrane wall. Continuous dialysis is achieved by maintaining a concentration gradient across the membrane. This gradient is achieved by the continuous flow of dialysate over the outside of the membrane wall.
In order to obtain maximum efficiency from a given coil dialyzer under given flow conditions through the casing it is necessary to prevent any shunting or bypass of the dialysate fluid around the permeable tubing. However, most coil type dialyzers have a substantial amount of space between the last outside layer of membrane tubing and the casing wall where such shunting occurs. This is because there is usually one or more layers of support material on top of the last permeable membrane layer which provide a path of reduced flow resistance to the incoming dialysate.
Shunting of dialysate through these layers of support material causes a decrease in the efficiency of the dialyzer. In coil type artificial kidneys, the portions of the dialysate fluid flowing through these layers increases with time up to a certin maximum value.